Memory circuits are at the core of any computer system. In this regard, one of the driving forces in memory circuit design has been to place a maximum number of memory cells in as small a physical space as possible. Initially, memory circuit design was constrained by the planar nature (i.e., two-dimensional) of the physical substrates upon which memory cells were fabricated. This constraint has given way to three-dimensional spaces that include the area provided by the physical substrates and the space above and below such substrates. As such, memory circuit designers introduced vertical memory cells resulting in three-dimensional memory arrays.
Vertical memory cells have themselves evolved into various forms. For example, U.S. Pat. No. 6,034,882 to Johnson et al., which is hereby fully incorporated by reference, describes a vertically fabricated memory cell having a steering element for enhancing the flow of current in one direction and a state change element for retaining a programmed state. The steering element functions similar to a diode and conducts more readily in one direction than in the other. In this regard, a current can be forced into the state change element for programming or for sensing the state thereof.
Additionally, U.S. pending application Ser. No. 09/560,626, and its continuation-in-part, U.S. patent application Ser. No. 09/814,727, both assigned to the assignee of the instant application, and titled “Three-dimensional Memory Array and Method of Manufacture,” and which are both hereby also fully incorporated by reference, describe a multi-level memory array having a plurality of rail-stacks forming memory cells. As such, the various material layers of the rail-stacks are configured to perform diode functions by conducting current in one direction more readily than in the other. This function facilitates programming of the memory cells and sensing of the states thereof.
In this regard, the diode functions accomplished by the above-described memory designs are typically performed by two layers of a single semiconductor material such as, for example, silicon (Si)—but with each layer having an opposite conductivity dopant (e.g., n-type silicon and p-type silicon). Such structures have been commonly referred to a homo-structures or homo-junctions because a single semiconductor material is used. In terms of ease of fabrication, relying on a single semiconductor material such as Si, SiGe, GaAs, or InP, for accomplishing the diode functions is advantageous. However, relying on a single semiconductor material for these functions also places certain constraints on the achievable device performance. For example, physical dimensions and doping concentrations can be varied to increase device performance, but only to a limit before other device characteristics begin to decrease or fail.
Hence, materials and methods for allowing higher achievable device performance such as, for example, the diode function of memory cells, is highly desirable.